Paints applied to substrates are cured and dried by means of UV cross-linking. The paint remains fluid until it is cross-linked and can be abruptly hardened after application on a substrate.
As the UV radiation source, in most cases, a radiation source is used which emits both UV radiation and visible light and a high percentage of infrared radiation (IR radiation). The high percentage of IR radiation leads to high processing temperatures which can damage the substrates to be irradiated. A mercury-vapor lamp can be mentioned as an example of such a radiation source. In particular if the painted substrates are plastic substrates, critical temperatures are often reached if the conventional irradiation is used. For the plastic material known as ABS, for example, temperatures of 85° C. should not be exceeded.
However, during the cross-linking of such paints, two competing effects have to be reconciled: on the one hand, such surfaces should be as resistant to scratching as possible. Particularly in the field of the automotive industry, resistance to micro-scratching, for example in the context of the resistance against car wash facilities, is a recurring issue. On the other hand, however, the layers have to have good adherence to the surface.
For components applied in the exterior area of vehicles, such as trim strips or radiator grilles, it is often desirable to have a metallic look. However, these components are often based on plastic substrates, which are provided with a metallic look by means of coating. According to a very promising method, first a paint layer (basecoat) is applied to these substrates, among other things, to smooth the surface. Subsequently, one or more mainly metallic layers are applied, such as by means of PVD (physical vapor deposition). These layers give the component its metallic look. Hereafter, the PVD layer is covered in high-gloss or mat finish by means of an essentially transparent UV-curable paint (topcoat).
On the one hand, the outer paint layer (topcoat) now has to have the above-mentioned resistance to micro-scratching.
This can be achieved, for example, by means of an increased degree of cross-linking. A drawback of this is, however, that the increased cross-linking of the paint layers leads to an increased brittleness caused by, among other things, increased shrinking during polymerization.
On the other hand, the topcoat and the PVD layer have different coefficients of thermal expansion. If, for example, the temperature changes or mechanical stresses occur, when the underlying material changes its extension corresponding to a different coefficient of thermal expansion, the above-mentioned increased brittleness with stronger cross-linking, has the effect that the paint no longer adheres on the material and thus flakes. In particular also in the context of stone chipping, such a brittle paint can additionally lead to fatal flaking of the paint.
It is thus desirable to provide a paint having good resistance against micro-scratching without being brittle.